Effects of the Femoral Offset and the Head Size on the Safe Range of Motion in Total Hip Arthroplasty

The purpose of this study was to quantify the effects of femoral offset and head size on range of motion (ROM) after total hip arthroplasty. Modular prostheses were implanted into 11 cadaveric hips using a posterolateral approach and tested for ROM with 3 different offsets and 5 different femoral head sizes. Increasing the femoral offset to 4 and 8 mm resulted in 21.1° and 26.7° of improved flexion, and 13.7° and 21.2° of improved internal rotation, respectively. The ROM improved in a head size–dependent manner primarily because of increasing the jumping distance of the femoral head rather than delaying any impingement. In contrast, the effectiveness of femoral offset was driven by delayed osseous impingement.